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C. LUSTY, W.A.N.AMARAL, W. D.HAWTHORNE, L.T. HONG & S. OLDFIELD (2007)
THE CASE OF TREE SPECIES
Between 1995 and 1998 a Dutch Government-funded project undertaken by the World
Conservation Monitoring Centre and the IUCN SSC assessed 10,000 tree species according
to the 2.3 version of the IUCN categories, of which 5999 were threatened and documented in
the World List of Threatened Trees (Oldfield et al. 1998). As part of the project William
Hawthorne reviewed the 2.3 version of the IUCN categories and suggested several rules of
thumb for the application of the criteria and associated definitions; many of which were taken
up and presented as guidelines to the several hundred assessors involved. One of the main
recommendations that arose from his review was that assessing tropical tree species according
to their distribution (i.e. criterion B) is the most appropriate and practical way of optimizing
use of available data; by contrast “approaches via notions of population size or change are
likely to be unreliable or untenable” (Hawthorne 1995).
Examples of rules of thumb used in the assessment of trees include:
– defining mature individuals as those which have reached potential according to their
ecological niche – canopy species which have reached the canopy etc.;
– estimating generation length to be 10–20 years for medium-large pioneer trees, 50 years
for most tropical species and 100 years for slow-growing species;
– measuring EOO for tropical species using degree squares (i.e. slightly more than 100 km
square) and a finer resolution for higher threat categories
More than half the assessments of threatened tree species fulfilled the B criterion and were
assigned the ‘Vulnerable’ category (Table 1). Many of these assessments were ‘inferred’ from
declines in habitat. These are tree species usually from restricted areas of forest type habitats,
which have declined by at least 20% in the past 100 years (i.e., approximately 2–3 tree
generations). The lack of data may have precluded more severe threat categories from being
assigned; available data to estimate population size were very rare and those species that were
assessed using the C criterion were usually highly specified or confined to islands or mountains.
Very few classifications were listed under more than one criterion.
Table 1. The assessment of tree species using the 2.3 version of the IUCN categories
A criterion B criterion C criterion D criterion Total
Declining Population Less than 10,000 Population
population of at confined to individuals and confined to 100
least 20% 20,000km 2 and declining km 2 or 5
declining
locations
% tree species 22% 56% 6% 16%
assessed
Number of 1320 3359 360 960 5999
tree species
assessed
Ex CR EN VU DD
% tree species 2% 16% 22% 60% 5%
assessed
Number of tree 95 976 1,319 3,609 375
species assessed
Source: Oldfield & Lusty (1998)
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